It is almost a decade since he first announced his ambition to occupy America's highest office, but whatever the result of today's US presidential vote, history will probably judge that John McCain's fate was sealed by a 15-minute meeting in March.

That was when he first met Alaskan Governor Sarah Palin. A two-hour phone call followed six months later, then a hastily arranged meeting at his Arizona ranch where he asked her to be his vice-presidential running mate.

That choice - one of the most spectacular gambles in US political history - has come to define the McCain campaign. And despite being a national hero, he goes into today's election a clear underdog.

Senator McCain at 72 is the oldest presidential hopeful selected to run by a major party. His selection of Mrs Palin shocked many and turned several respected Republicans against his cause, most notably former secretary of state Colin Powell.

But those who have known him during his two decades in the Senate say it was classic McCain - highlighting his willingness to gamble, as well as his fighting instincts.

Before picking Mrs Palin, his campaign advisers were giving him nothing but bad news.

The selection of Mrs Palin, who at 44 is three years younger than Barack Obama, offered intriguing possibilities, despite the risks. She is only the second woman nominated for the vice-presidency. A reformer who had taken on the Alaskan Republican establishment, she would burnish Senator McCain's reputation as an outsider, unafraid to speak his mind. She was also an evangelical Christian with deeply conservative views - adding appeal to a Republican base that had seemed lukewarm to a McCain presidency. And her 'you betcha' hockey mum image provided a populist counterbalance to Senator Obama's charisma.

On the one hand, the adoring crowds who mob her at every event prove she has energised the Republican base. On the other, her lack of experience - glaringly exposed during rare interviews - turned off some independents.

But perhaps the most striking aspect of Mrs Palin's selection is the way it has radically re-shaped Senator McCain's campaigning style.

While they have attempted to portray themselves as two mavericks ready to shake up Washington, Senator McCain has been seemingly transformed into an establishment Republican figure.

Across America, you hear people talking about Senator McCain pandering to the conservative base - rather than staking a claim based on his own credentials.

'I've always liked the man, he's one of the good guys,' Mitch Miller, a fellow navy veteran, said in Colorado. 'He's a real American hero who has never been afraid to speak his mind, but right now I just don't recognise him. I just don't know why he's done what he's done.'

The last month of the McCain campaign has been less about his own considerable strengths and more about attacks on his opponent. He has labelled Senator Obama as an empty orator with 'dangerous' associations and 'socialist' tendencies, unleashing Mrs Palin as his main attack dog. With advisers steeped in the successful strategies of Karl Rove - the strategy guru who helped George W. Bush win two elections by mobilising the Christian right - he appears to have struggled to mould his own campaign.

Against an opponent who is reaching independents and new voters - the young and minorities - polls suggest that connecting with the Republican right will not be enough.

Voters who once respected Senator McCain saw in him as a tough, honest broker who was not afraid to stand apart from his own party. This was the candidate who had taken knocks, who had lost his lead in the 2000 primaries to Mr Bush, defeated in part by a racially tinged whispering campaign about one his daughters, who was adopted from Bangladesh.

That was the McCain who stood on principle - the senator who skirted ideological excesses to forge bipartisan deals to cut unnecessary spending, reform campaign finance and push alternative energy.

This time around, he has struggled to sell policies that seek to cement Mr Bush's tax cuts to America's wealthiest firms.

His principles have not always won him friends during two decades in the Senate - but they have won him respect. They pointed to an inner strength forged during six years in Vietnam's prisoner of war camps after his jet fighter was shot down over Hanoi in 1967.

Despite the psychological and physical scars - he still cannot raise his arms above shoulder height - he helped lift a crippling economic embargo on Vietnam and normalise Washington's relations with Hanoi.

While Senator McCain has frequently sneered at his rival's eloquence, he is capable of delivering powerful speeches, particularly when he melds his politics with his life's lessons. An example: 'I fell in love with my country when I was a prisoner in someone else's. I loved it not just for the many comforts of life here. I loved it for its decency, for its faith in the wisdom, justice and goodness of its people.

'My country saved me, and I cannot forget it.'

Senator McCain was speaking as he accepted his party's presidential nomination, an event which had a few months earlier looked impossible as his primary effort collapsed.

As extraordinary as his comeback was, greater struggles lay ahead.

It was clear that Senator McCain's ambitions were up against an unprecedented challenge in Senator Obama. He has found himself on the wrong side of history. If he and Mrs Palin are to win, it will be against that tide.